Transcript Resistance to Frequency Converter

Resistance to Frequency
Converter
Amol Mupid
Andrew Ricketts
Outline
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Original design
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Modified design
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Component parts
Roadblock
Buffer optimization
Design specification
Conclusion
Chemiresistive Sensors
Rs 
1
AC
Rs : resistance of nanowire
C : concentration of the gas
Rs
A,α : constants that change with
type of gas and temperature
400
350

Original design
R3
C
Vout
D
Q
R2
+
R1
+
CLK
-
R4
Rs
R5
Qbar
Original design components
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Diode
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Spice simulation
…but layout issues insurmountable
Original design components
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Zener Diode
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More simulations possible
…but layout even more challenging
Modified design
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Key point is that what is desired is a
way to control oscillations based on
input voltage

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Voltage Controlled Oscillator (VCO)
Buffer added to output to ensure rapid
rise and fall of output
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square wave
VCO design
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LC tank oscillators
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Good phase noise with low power
but tuning range is relatively low
Output frequency may fall out of range due to
process variations
Spiral inductors occupy a lot of area, high cost and
low yield issues.
Ring oscillators
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Easy integration, high yield, low cost.
Less chip area
In-phase outputs
Single delay cell
Schematic of rectified VCO
Buffer optimization
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Initial single stage buffer
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Moved output close to binary
Had difficulty clamping small swings about
origin
Double stage buffer
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Delay increase inconsequential
Greatly improved clamping range
Layout of complete design
Period versus Vin change
Transistor sizing
Transistor
Width (um)
Length (um)
Mb1
11.0
0.6
Mp1,Mp2
5.16
0.6
Mp3,Mp4
5.16
0.6
Mn1,Mn2
3.0
0.6
MpInv
18.0
0.6
MnInv
6.0
0.6
Core area 48.75 X162.6 = 7,926(um^2)
Voltage dependant output
periodicity
3
2.5
2
3.3V
3.5V
4V
4.5V
5V
1.5
1
0.5
0
period
(NS)
frequency
(GHz)
Power dissipation ( mW)
30
25
20
3.3v
3.5V
4V
4.5V
5V
15
10
5
0
max power
avg power
rms power
Resistance
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R
Vcontrol
Rs
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We want the Vcontrol be to
be in between operable
range
=> Rs*VDD/ (Rs+ R) has to
be in between 3.3V and 5V
Resistance
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For max swing when Rs only increases
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For max swing when Rs increases and
decreases
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R = (1.7 * Rs)/3.3
R = (1.1 * Rs)/3.9
For max swing when Rs only decreases
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R = (0.5 Rs)/4.5
Layout with pads
Layout simulation
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R = 11 KΩ
Rs = 25 KΩ
Per = 1.9 ns
Layout simulation
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R = 11 KΩ
Rs = 40 KΩ
Per = 2.35 ns
Layout simulation
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R = 11 KΩ
Rs = 99 KΩ
Per = 2.85 ns
Resistance (K ohm)
Layout linearity (almost)
120
100
80
60
40
20
0
1.8
2.3
Period (ns)
2.8
Conclusion
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The change in the sensor resistance
can be detected in “ns” range and
converted to square wave pulses
This completely eliminates the need of
ADC, huge potential resource savings.
Successfully overcame practical design
issues and produced desired results.
Thank You
Questions??